/Engine/Source/Runtime/Engine/Classes/Engine/DemoNetDriver.h
Simplified rating of replay frame fidelity as percentage of actors that were replicated. [0..1] where 0 means nothing was recorded this frame and 1 means full fidelity. This treats all actors equally. Assuming more important actors are prioritized higher, in general actual "fidelity" is expected to be higher than reported, which should be fine for detecting low-fidelity frame/intervals in replay file.
float GetLastReplayFrameFidelity() const